The present invention relates generally to communication systems and in particular to soft handoff combining hybrid automatic repeat request (ARQ).
Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a cellular system. In a CDMA system, a mobile station communicates with any one or more of a plurality of base stations dispersed in a geographic region. Each base station continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a base station will be simply referred to as a pilot. The mobile station monitors the pilots and measures the received energy of the pilots.
IS-95 defines a number of states and channels for communication between the mobile station and the base station. For example, in the Mobile Station Control on the Traffic State, the base station communicates with the mobile station over a Forward Traffic Channel, and the mobile station communicates with the base station over a Reverse Traffic Channel. During a call, the mobile station must constantly monitor and maintain four sets of pilots collectively referred to as the Pilot Setxe2x80x94the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set are pilots associated with the Forward Traffic Channel assigned to the mobile station. The Candidate Set are pilots that are not currently in the Active Set but have been received by a particular mobile station with sufficient strength to indicate that the associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set are pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set are all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
The mobile station constantly searches a Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. The mobile station signals this event to the base station using the Pilot Strength Measurement Message. As the mobile station moves from the region covered by one base station to another, the base station promotes certain pilots from the Candidate Set to the Active Set, Neighbor Set to the Candidate Set, and notifies the mobile station of the promotions via a Handoff Direction Message. When the mobile station commences communication with a new base station in the new Active Set before terminating communications with the old base station, a xe2x80x9csoft handoffxe2x80x9d has occurred. For the reverse link, typically each base station demodulates and decodes each frame or packet independently. It is up to the switching center to arbitrate between the two base station""s decoded frames. Such soft-handoff operation has multiple advantages. Qualitatively, this feature improves and renders more reliable handoff between base stations as a user moves from one cell to the adjacent one. Quantitatively soft-handoff improves the capacity/coverage in a CDMA system.
The CDG Phase-II (consortium of service providers that have set requirements for the next phase of wireless communications) requires a high peak rate ( greater than 1 Mbps) as well as higher average throughput (approximately 600 kbps) on the reverse link. To achieve these requirements on the reverse link techniques such as hybrid ARQ and Adaptive Modulation and Coding are needed.
Adaptive Modulation and Coding (AMC) provides the flexibility to match the modulation and forward error correction (FEC) coding scheme to the average channel conditions for each user. AMC promises a large increase in average data rate for users that have a favorable channel quality due to their proximity to the base site or other geographical advantage. Enhanced GSM systems using AMC offer data rates as high as 384 kbps compared to 100 kbps without AMC. Likewise, 1.25 MHz CDMA systems can offer downlink peak data rates as high as 5 Mbps through AMC, where 460 kbps was typical without AMC. AMC, however, does have a few drawbacks. AMC is sensitive to measurement error and delay. In order to select the appropriate modulation, the scheduler must be aware of the channel quality. Errors in the channel estimate will cause the scheduler to select the wrong data rate and either transmit at too high a power, wasting system capacity, or too low a power, raising the block error rate. Delay in reporting channel measurements also reduces the reliability of the channel quality estimate due to constantly varying mobile channel. To overcome measurement delay, the frequency of the channel measurement reports may be increased, however, the measurement reports consume system capacity that otherwise might be used to carry data.
Hybrid ARQ (H-ARQ) is an implicit link adaptation technique. Whereas, in AMC explicit C/I measurements or similar measurements are used to set the modulation and coding format, in H-ARQ, link layer acknowledgements are used for re-transmission decisions. There are many schemes for implementing H-ARQ, such as, Chase combining, Rate compatible Punctured Turbo codes and Incremental Redundancy. Incremental redundancy or H-ARQ-type-II is another implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ-type-III also belongs to the class of incremental redundancy ARQ schemes. However, with H-ARQ-type-III, each retransmission is self-decodable which is not the case with H-ARQ-type II. Chase combining (also called H-ARQ-type-III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received SNR. Diversity (time) gain is thus obtained. In the H-ARQ-type-II with multiple redundancy version different puncture bits are used in each retransmission. The details for how to implement the various H-ARQ schemes are commonly known in the art and therefore are not discussed herein.
H-ARQ combined with AMC can greatly increase user throughputs, potentially doubling system capacity. In effect, Hybrid ARQ adapts to the channel by sending additional increments of redundancy, which increases the coding rate and effectively lowers the data rate to match the channel. Hybrid ARQ does not rely only on channel estimates but also relies on the errors signaled by the ARQ protocol. Currently, the H-ARQ function resides at the BTS while the reverse link soft-handoff is performed at the RNC. Furthermore, base stations can communicate only through the RNC.
Thus there is a need for a method of combining AMC and H-ARQ scheme(s) with soft handoff in the reverse link to maximize system throughput and gain.